Noise variance estimator for speech enhancement
A speech enhancement method operative for devices having limited available memory is described. The method is appropriate for very noisy environments and is capable of estimating the relative strengths of speech and noise components during both the presence as well as the absence of speech.
1. A method for enhancing speech components of an audio signal composed of speech and noise components, comprising
transforming the audio signal from the time domain to a plurality of subbands in the frequency domain,
wherein each of said plurality of subbands is presumed to have a speech component and a noise component, said noise component having an amplitude and a variance at time index m, wherein said amplitude of the noise component is estimated by exploiting statistical differences that distinguish between the speech component and the noise component,
processing each of said plurality of subbands, said processing including applying a gain factor, wherein said gain factor is derived at least in part from an estimation of said variance in noise components, wherein the estimation comprises
at each time index m, updating said estimation of variance in noise components of the subband signal from an average of past estimates of the amplitude of noise components in the subband signal, and
wherein said past estimates of the amplitude of noise components in the subband signal having values greater than a threshold are excluded from or underweighted in said weighted average, and
transforming the processed subband signal from the frequency domain to the time domain to provide an audio signal in which speech components are enhanced.
2. A method according to claim 1 wherein the average of past estimates of the amplitude of noise components is a weighted average of the square of the past estimate of the amplitude of a noise component and the past estimated variance in noise components.
3. A method according to claim 2 wherein the weighting function of the weighted average is a preselected constant.
4. A method according to claim 1 wherein each estimate of the amplitude of noise components in the subband signal is a function of an estimate of variance in noise components of the subband signal, an estimate of variance in speech components of the subband signal, and the amplitude of the subband signal.
5. A method according to claim 1 wherein said threshold is a function of ψ(1+{circumflex over (ξ)}(m)){circumflex over (λ)} d (m), where {circumflex over (ξ)} is the estimated a priori signal-to-noise ratio, {circumflex over (λ)} d is the estimated variance in noise components of the subband signal, and ψ is a constant determined by a predetermined maximum amount of an estimation bias.
6. A method according to claim 5 wherein each estimate of the amplitude of noise components in the subband signal is a function of an estimate of variance in noise components of the subband signal, an estimate of variance in speech components of the subband signal, and the amplitude of the subband signal.
7. Apparatus adapted to perform the methods of any one of claims 1 through 6 .
8. A non-transitory computer-readable storage medium encoded with a computer program for causing a computer to perform the method of any one of claims 1 through 6 .